For example, you want to teleport players to a certain point if they go above y=200. To do this, we will use the dx/dy/dz cuboid volume selectors, which allow us to specify a width/height/length in which to match a player.
An incorrect first attempt to match players above y=200 might look like:
say @a[y=200,dy=500]
The problem with this is that, as they are not specified but y is, x and z will default to the x and z of the current executing position, which is in this case the thing running the command. So you’ll find that this works if you run it in chat (because it defaults to your x/z), but won’t work when you run it in a command block unless you have the same x/z as the command block (directly above/below it). Something similar is true for dx and dz: Because dy is specified, the other two default to 0. Which means the command will internally look like this (assume it’s run from a commandblock located at 1 2 3):
say @a[x=1,y=200,z=3,dx=0,dy=500,dz=0]
Instead, we’ll need to execute at the player, so that the x and z will default to the players own x and z, which they will always match:
execute as @a at @s run say @s[y=200,dy=500]
The same technique can be used to find players above/below any x/y/z coordinate.
To detect below a certain height, you can make the dy negative.
/execute at @a at @s run say @s[y=-30,dz=-500] this will detect any players below `-30`